Conventionally, conversion mechanisms such as crank mechanisms are broadly used for conversion between rotary motion and linear reciprocal motion. Even though reciprocal motion can be directly caused, driving in one direction and then driving in the reverse direction requires a stoppage in the course thereof, making it difficult to increase the speed. By converting a continuous rotary motion caused by a motor, etc. into a reciprocal motion, it is possible to easily obtain a fast reciprocal motion.
FIG. 11 shows a schematic structure of a cutter 1 requiring a fast reciprocal motion converted from a rotary motion for cutting. In the cutter 1, a cutting blade 6 is reciprocally moved at high speed at the cutting head 5 provided on a guide bridge 4, to clip a cut-part 8 out of a to-be-cut-sheet 7 on a holding surface 3 above a cutting table 2. Within the cutting head 5, rotary motion is converted into reciprocal motion. The holding surface 3 in general is rectangular in form. The guide bridge 4 has a form extending in a direction parallel with the shorter side of the cutting table 2, which is movable in both directions along the guides provided on the side edges on the longer sides. The cutting head 5 is movable in both directions along an extending direction of the guide bridge 4. The cutting blade 6 protrudes toward the holding surface 4 from the cutting head 5. The cutting blade 6 is reciprocally driven in a direction perpendicular to the holding surface 3 in a state to pierce through the to-be-cut-sheet 7 held on the holding surface 3, thus being allowed to reciprocally move at high speed and angularly displace about an axis parallel with the reciprocal direction.
The holding surface 3 of the cutter 1 is in a state planted with bristles of a material comparatively high in hardness, e.g. synthetic resin. Even in case the cutting blade 6 is inserted piercingly, the bristles deform in a manner avoiding the cutting blade 6 thereby being prevented from being cut. The guide bridge 4 and cutting head 5 clips a cut-part 8 out of the to-be-cut-sheet 7 according to the cutting data inputted to the cutter 1. For such a cutter 1, there is a need to increase the moving speed of the guide bridge 4 and cutting head 5 and further the reciprocation speed of the cutting blade 6 in order to improve the efficiency of cutting. In order to increase the moving speed, the cutting head 5 is necessarily reduced in size and weight. For increasing the reciprocal speed of the cutting blade 6, there is a need to increase the rotational speed on a drive source of within the cutting head 5. However, when converting a rotary motion into a reciprocal motion within the cutting head 5, vibration readily occurs due to the components, etc. of a motion caused resulting therefrom. Particularly, when reciprocal motion becomes high in speed, vibration increases.
When vibration is encountered in the occurrence of a reciprocal motion in the cutter, there is a fear of increasing the error in a contour cutting a cut-part 8 or causing a fatigue in various regions including the cutting blade 6 to be readily broken. In the crank mechanism, because rotary motion is converted into reciprocal motion while coupling the crank rod at its one end in a position eccentric from the rotary shaft and regulating the other end of the crank rod in a direction of reciprocal motion, offset load is applied to the rotary shaft. In case applying a load balanced with the load applied to the rotary shaft by the crank rod to the side opposite to the coupling to the crank rod sandwiching the rotary shaft, the reacting force driving the crank rod can be canceled to damp vibration (see JP-B-6-53358 and JP-B-7-279, for example). JP-B-6-53358 discloses a mechanism that a crank rod of a link mechanism for a counter weight compensating for a load on the crank mechanism is coupled to axial both sides of a crank mechanism for reciprocally moving the cutting blade, to reduce vibration by means of a counter weight provided on the opposite side to the crank mechanism, i.e. opposite by 180 degrees. JP-B-7-279 discloses a mechanism that a pair of rotary shafts having balance weights are arranged parallel with a crankshaft of a crank mechanism for reciprocally moving the cutting blade so that the balance weight is given a weight of a half load, to rotate the both balance weights reverse to the crank mechanism thereby reducing vibration.
The mechanism for converting rotary motion into reciprocal motion is used on the sewing machine. As to sewing machines, there are disclosures of vibration reducing mechanisms (see JP-A-6-154459 and JP-A-7-124361). In JP-A-6-154459, a pair of balancer shafts are arranged above a crankshaft of a needle crank mechanism for reciprocally driving the needle of a sewing machine, to rotate forward and reverse eccentric balancers at equal speed and reverse in direction to each other thereby reducing vibration. In JP-A-7-124361, a pair of balancer shafts are arranged on both sides sandwiching a needle crankshaft of a sewing machine, to provide the needle crank itself with an eccentric center of gravity to have a balance with a load wherein further the balancer shafts in one pair are provided with balancers and rotated reverse to the needle crankshaft thereby reducing vibration.
In case using a balancer that is a counter weight having a center of gravity in a position eccentric relative to the rotary shaft, compensation is possible for a static load. However, on the mechanism for converting rotary motion into reciprocal motion, e.g. a crank mechanism, load varies dynamically even at a constant rotational speed. Accordingly, in case attempted to cancel a load encountered upon converting rotary motion into reciprocal motion by providing a balancer on the rotary shaft, it cannot be canceled perfectly. In JP-A-6-154459, the needle is moved vertically through one crankshaft. Because a vibration damping apparatus mechanism does not operate symmetrically left and right about the axis of the needle, balance cannot be obtained perfectly in the horizontal direction. Even in case the crankshaft balancer and the rotary shaft balancer are rotated reverse by providing rotary shafts rotating reverse to the crank shaft on both sides sandwiching the crankshaft as in JP-B-7-279 or JP-A-7-124361, the balancer shafts must be arranged on both sides by being juxtaposed with the crankshaft. This increases the width of the cutting head, etc. thus making size reduction difficult.
In case providing such a link mechanism as to compensate for a load based on the crank mechanism on the side opposite, sandwiching the crankshaft, to the direction that reciprocal motion is caused by the crank mechanism as in JP-B-6-53358, a cutting head that protrudes a cutting blade downward requires a space for a link mechanism in the above thereof thus resulting in an increased height of the cutting head. Meanwhile, because the cranks are opposed at 180 degrees, in case the link mechanism is coupled to one lateral direction of a crank axis, a couple of forces occur at axial front and rear of the crank.